Google the words "Sex Pistols Lesser Free Trade Hall 1976", or simply "4 June 1976", and you can use the resultant 22,600,000 pieces of information to piece together a crudely helpful history of a) Manchester music, b) the birth of indie music and c) the "greatest gig of all time" that "changed music for ever". The fact that if you Google the additional words "swear I was there" you come across more details about that than the Sex Pistols' performance emphasises the show's reputation. Not least because – and this has become an integral element in the ensuing mythologising of the gig – there weren't that many people who'd bought the 60p ticket, but thousands now claim they did. Those who like to nourish the legend favour an estimate of around 40; other less romantic minds suggest a number closer to 100.

I was there. I was a witness, although not enough of one to notice at the time that what was taking place was "history". I had no idea I would talk and write about the gig for what is turning out to be the rest of my life, finding new ways to point out that the evening was something of a revelation because it instantly suggested that a) there were other people interested in music who made you feel, think and want to do or be something radical or individual, b) you could make music without the usual support systems of London record companies, promoters and showbiz managers, and c) here was an exciting way to assassinate Emerson Lake and Palmer, who indifferently perpetuated various demoralising forms of alienation, elitism, pomposity and complacency.

I'd gone on my 19-year-old own. I'm not sure what I actually recall or what I filled in using data acquired later as the gig was talked up into legend. We, the yokel audience, were scruffy, isolated avant garde music fans motivated to constantly search out new music. Many audience members have since become well known. So well known it appears now that the show was attended by a host of rock celebrities – members of Joy Division, New Order, the Fall, the Smiths, A Certain Ratio, Ludus, Simply Red, Buzzcocks, Magazine, the producer Martin Hannett. It was, in fact, attended by unassuming nonentities drawn to the gig from within a 20-mile radius of Manchester city centre perhaps because they were extremely frustrated by their stranded nonentity status, and craving purpose.

I seem to recall no one looked as though they were in a group, and they never would be, because of course at the time no one who looked a little ordinary, even dull, formed pop groups. This was to change, quite quickly, because the Sex Pistols themselves did not as such look like a band.

Would another group of 40 people have ended up forming the kind of groups that got formed because, suddenly, there appeared these perverse educators, these militant cultural critics ? Or was this obviously the 40 or so who would end up forming those bands and labels – and writing these words/taking photographs/designing sleeves/managing – because they attended the gig in the first place, and had agitating within them all those ideas, and just needed some sort of cabalistic psychic trigger? (And what happened to those of the 40 that did not form bands, etc? The myth does not allow these gaps to be filled in.)

Six weeks later, on 20 July, the Pistols returned, stronger, faster, harder, darker, officially notorious. The Lesser Free Trade Hall was now full of more knowing fans – already with shorter hair and narrower trousers and an edge more sectarian self-confidence – paying a pound. There was, relatively speaking, someone already famous at this show, passionate local TV personality Tony Wilson, obsessed with Manchester's pioneering and progressive credentials. Mostly, though, it was still a crowd of unknowns, including the reserved out-of-towner Ian Curtis, meeting people he had something in common with and totally ready to let the ordinary but uncanny Rotten inspire him.

Because, as is obvious once time has passed, one surprising thing leads to another. The first Pistols show led to the second Pistols show led to Wilson's experimental pop TV show So It Goes, to Buzzcocks' Spiral Scratch, to the endless Fall, to Factory Records, to the Hacienda, to Marr and Morrissey, to Madchester, to the Stone Roses at Spike Island, to Oasis, to a postmodern brand of civic pride, and, what with one thing and another, to I'm a Celebrity Get Me Out of Here.

The momentum caused by the event has now, perhaps, died down, or paused for thought. Or, ultimately, the momentum has turned into a constant nostalgic commentary on the momentum itself – what caused it, how we remember it and what happened because of it to Manchester and its regenerated sociocultural history.